Polyolefin material having a low density

Abstract

A polyolefin material that is formed by solid state drawing of a thermoplastic composition containing a continuous phase that includes a polyolefin matrix polymer and nanoinclusion additive is provided. The nanoinclusion additive is dispersed within the continuous phase as discrete nano-scale phase domains. When drawn, the nano-scale phase domains are able to interact with the matrix in a unique manner to create a network of nanopores.

What is claimed is: 
     
       1. A polyolefin material that is formed by drawing of a thermoplastic composition, wherein the thermoplastic composition contains
   a continuous phase that includes a polyolefin matrix polymer and   from about 1 wt. % to about 20 wt. %, based on the weight of the continuous phase, of a polymeric microinclusion additive dispersed within the continuous phase in the form of discrete domains;   from about 0.05 wt. % to about 20 wt. % based on the weight of the continuous phase, of a polymeric nanoinclusion additive dispersed within the continuous phase in the form of discrete domains;   
 wherein a porous network is defined in the composition that includes a plurality of nanopores having an average cross-sectional dimension of about 800 nanometers or less, and a plurality of micropores having an aspect ratio of an axial dimension to a cross-sectional dimension of about 1 to about 30, the composition having a density of about 0.90 g/cm 3  or less; and 
 wherein the polyolefin material exhibits a hydrohead value of about  50  centimeters or more. 
 
     
     
       2. The polyolefin material of  claim 1 , wherein the nanopores have an average cross-sectional dimension of from about 5 to about 700 nanometers, an average axial dimension of from about 100 to about 5000 nanometers, or a combination thereof. 
     
     
       3. The polyolefin material of  claim 1 , wherein the polyolefin matrix polymer has a melt flow rate of from about 0.5 to about 80 grams per 10 minutes as determined at a load of 2160 grams and at 230° C. in accordance with ASTM D1238. 
     
     
       4. The polyolefin material of  claim 1 , wherein the polyolefin matrix polymer is a propylene homopolymer, propylene/α-olefin copolymer, ethylene/α-olefin copolymer, or a combination thereof. 
     
     
       5. The polyolefin material of  claim 1 , wherein the polyolefin matrix polymer is a substantially isotactic polypropylene homopolymer or a copolymer containing at least about 90% by weight propylene. 
     
     
       6. The polyolefin material of  claim 1 , wherein the continuous phase constitutes from about 60 wt. % to about 99 wt. % of the thermoplastic composition. 
     
     
       7. The polyolefin material of  claim 1 , wherein the nanoinclusion additive is a functionalized polyolefin. 
     
     
       8. The polyolefin material of  claim 7 , wherein the functionalized polyolefin is a polyepoxide. 
     
     
       9. The polyolefin material of  claim 7 , wherein the nanoinclusion additive has melt flow rate of from about 0.1 to about 100 grams per 10 minutes, determined at a load of 2160 grams and at a temperature at least about 40° C. above the melting temperature in accordance with ASTM D1238. 
     
     
       10. The polyolefin material of  claim 7 , wherein the ratio of the melt flow rate of the polyolefin to the melt flow rate of the nanoinclusion additive is from about 0.2 to about 8. 
     
     
       11. The polyolefin material of  claim 1 , wherein the nanoinclusion additive is in the form of nano-scale domains having an average cross-sectional dimension of from about 1 nanometer to about 1000 nanometers, the microinclusion additive is in the form of micro-scale domains having an average axial dimension of from about 1 micrometer to about 400 micrometers, or a combination thereof. 
     
     
       12. The polyolefin material of  claim 1 , wherein the polymer of the microinclusion additive is polylactic acid. 
     
     
       13. The polyolefin material of  claim 1 , wherein the polymer of the microinclusion additive has a glass transition temperature of about 0° C. or more. 
     
     
       14. The polyolefin material of  claim 1 , wherein the microinclusion additive has a melt flow rate of from about 5 to about 200 grams per 10 minutes, determined at a load of 2160 grams and at a temperature of 210° C. 
     
     
       15. The polyolefin material of  claim 1 , wherein the ratio of the melt flow rate of the microinclusion additive to the melt flow rate of the polyolefin matrix polymer is from about 0.5 to about 10. 
     
     
       16. The polyolefin material of  claim 1 , wherein the ratio of the Young's modulus elasticity of the polyolefin matrix polymer to the Young's modulus of elasticity of the microinclusion additive is from about 1 to about 250. 
     
     
       17. The polyolefin material of  claim 1 , wherein the total pore volume of the polyolefin material is from about 15% to about 80% per cubic centimeter. 
     
     
       18. The polyolefin material of  claim 1 , wherein nanopores constitute about 20 vol. % or more of the total pore volume of the polyolefin material. 
     
     
       19. The polyolefin material of  claim 1 , wherein the thermoplastic composition: has a density of about 0.85 g/cm 3  or less, is generally free of blowing agents, or is generally free of pore-initiating inorganic oxide fillers. 
     
     
       20. An absorbent article comprising a polyolefin material according to  claim 1 , wherein the absorbent article includes a liquid-impermeable layer, liquid-permeable layer, and an absorbent core. 
     
     
       21. The absorbent article of  claim 20 , wherein the liquid-impermeable layer includes the polyolefin material, or wherein the liquid-impermeable layer includes a nonwoven web material laminated to a film, and wherein the film includes the polyolefin material. 
     
     
       22. A method for forming a polyolefin material, the method comprising:
 forming a thermoplastic composition that contains
 a continuous phase that includes a polyolefin matrix polymer and 
 from about 1 wt. % to about 20 wt. %, based on the weight of the continuous phase, of a polymeric microinclusion additive dispersed within the continuous phase in the form of discrete domains; 
 from about 0.05 wt. % to about 20 wt. % based on the weight of the continuous phase, of a polymeric nanoinclusion additive dispersed within the continuous phase in the form of discrete domains; and 
 
 solid state drawing the thermoplastic composition, wherein a porous network is defined in the composition that includes a plurality of nanopores having an average cross-sectional dimension of about 800 nanometers or less and a plurality of micropores having an aspect ratio of an axial dimension to a cross-sectional dimension of about 1 to about 30, the composition having a density of about 0.90 g/cm 3  or less; and 
 wherein the polyolefin material exhibits a hydrohead value of about 50 centimeters or more. 
 
     
     
       23. The method of  claim 22 , wherein the thermoplastic composition is stretched to a draw ratio of from about 1.1 to about 3.0.